1. Field of the Invention
The present invention relates to an automatic tracking process of the joint bevel and the relative equipment for the welding of coaxial pipes by means of an operation with which two pipes are joined with several runs of welding material, so as to form a material continuity without "side of lack fusion" defects.
More specifically, this equipment comprises a governing unit based on a miniprocessor in which sample values of electrical parameters are inserted to compare them with the parameters of voltage, current and voltaic arc impedance values, in order to generate command signals which activate and pilot the driving source programmed for the variations in the position of an oscillating welding torch so that the run of welding material is basically deposited at the center of the throat of the joint bevel formed by the counterfacing coaxial pipe ends.
In the following description and claims the term "joint bevel" indicates the space or throat which appears with the "bevelling" of the appropriate pipe ends of the counterfacing butts of the coaxial pipes to be welded, and the term "pipe" or "piping" indicates any substantilly circular-shaped tubing.
2. Discussion of the Background
Orbital equipment is known in the art, for the welding process with the well-known applicative "GMAW" (gas metal arch welding) method; in particular the "MAG" or "MIG" (metal active gas, or metal inert gas) versions. This known equipment essentially consists of a circular track which is fitted and blocked on the piping and at least one movable welding trolley which moves at a controlled rate along the track circling around the piping. The welding trolley, which preferably moves in both rotatory directions, is equipped with locking and sliding wheels on the track, a notched driving pinion which engages with a corresponding notched edge of the track itself for the orbital movement of the trolley and at least one oscillating welding torch of the continuous wire type.
The welding technique for pipes with orbital movement consists of depositing, by means of repeated passes of the welding trolley, a series of superimposed welding runs in the joint bevel. For this purpose the track is positioned near the joint bevel, the welding trolley is attached to this, and the voltaic arc of the torch of the welding trolley is in line with the axis of the joint bevel.
Owing to the "bevelled" shape of the joint bevel, the quantity of welding material gradually increases as the torch moves, with each pass, further away from the pipe axis. The torch, which is initially fixed with respect to the trolley, is consequently subjected to an oscillating movement whose extent, frequency and stasis (stoppage after each oscillation) vary both in the passage from one welding run to another and in effecting the same welding run in relation to the position of the trolley on the circumference of the pipe due to the influence of gravity on the welding metal in its liquid state. The orbital movement rate of the trolley and feeding rate of the welding wire also depend on the position of the trolley on the circumference of the pipe and these parameters, as also the extent, frequency and stoppage of the oscillation of the torch, in present orbital welding equipment, are controlled and piloted using sensors of the mechanical, magnetic, optical or "laser" type.
These solutions have proved to have disadvantages for the butt welding of pipe-lines, especially pipelines having a considerable wall thickness. In particular there is the formation of "side of lack fusion," which is typical of cases in which the welding pass is not centered and melts only one side of the joint bevel leaving the other unmelted. The resolution degree of the tracking system required for automatized systems, i.e. the capacity of the system of maintaining the welding pass in the center of the joint bevel thus avoiding the formation of "side of lack fusion" defects, is very high and is more or less about a tenth of a millimeter, or at the most two tenths of a millimeter (from .+-.0.1 to .+-.0.2 mm). The resolution degree of a sensor of the mechanical type is of about 5 tenths of a millimeter (.+-.0.5 mm) and therefore, as an absolute value, too high with respect to the quality of welding required (.+-.0.1-0.2 mm). The mechanical sensor, moreover, is connected to the welding torch by means of a bracket which in practice accentuates the parallelism error between the axis of the welding torch and the axis of the joint bevel. This error is also evaluated as being more or less 5 tenths of a millimeter (.+-.0.5 mm).
The industrial solutions of the known art which operate with the use of sensors of the magnetic, optical or "laser" type, have a better resolution degree than those which adopt a mechanical sensor, but also require a connection bracket to the welding torch which still generates a higher error than the quality requested. It is evident however that the correct performance of the welding process absolutely depends on the exact piloting of the mechanical units forming the welding trolley which must also be equipped with units capable of ensuring the correct oscillation of the torch and the correct feeding of the welding wire. In particular, the movement of the trolley must not be influenced by the diameter of the track and position of the trolley on the circumference of the pipe. Similarly the mechanical units which govern the feeding of the welding wire must exert an effective traction on the wire itself to be able to accelerate or decelerate the feeding without there being any slippage between the wire and the traction device. In the same way, the units which govern the oscillation of the welding torch must ensure the correct functioning of the oscillation itself in terms of width, frequency and stasis.